I would just like to add one step to Ernes notes. Anytime after you power down the host (step 3) and before you restart host (step 6), unpresent the LUNs from the host to the CX300.
This could be as simple as removing the host from the storage group on the CX300 or as I prefer, disable the corresponding zones. I prefer this over removing the host from the storage group so the LUNZ's don't show up (and easier than disabling ArrayCommPath). Then again, the LUNZ's are (for the most part) harmless.
Then again, I'm probably just stating the obvious as we don't want both sets of LUNs (now with the same disk signatures) presented simultaneously.